1. Field of the Invention
The present invention relates to an over-write capable magnetooptical recording method and a recording apparatus used therefor.
2. Related Background Art
In recent years, many efforts have been made to develop an optical recording/reproduction method which can satisfy various requirements including high density, large capacity, high access speed, and high recording/reproduction speed, and a recording apparatus, a reproduction apparatus, and a recording medium used therefor.
Of various optical recording/reproduction methods, the magnetooptical recording/reproduction method is most attractive due to its unique advantages in that information can be erased after use and new information can be repetitively recorded/reproduced.
A recording medium used in the magnetooptical recording/reproduction method has a perpendicular magnetic anisotropy layer or layers as a recording layer. The magnetic layer comprises, for example, amorphous GdFe, GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like. Concentric or spiral tracks are normally formed on the recording layer, and data is recorded on the tracks. In this specification, one of the "upward" and "downward" directions of the magnetization with respect to a film surface is defined as an "A direction", and the other one is defined as a "non-A direction". Data to be recorded is binarized in advance, and is recorded by two signals, i.e., a bit (B.sub.1) having an "A-directed" magnetization, and a bit (B.sub.0) having a "non-A-directed" magnetization. These bits B.sub.1 and B.sub.0 correspond to "1" and "0" levels of a digital signal, respectively.
In general, the direction of magnetization of the recording tracks can be aligned in the "non-A direction" by applying a strong bias field. This processing is called "initialization". Thereafter, a bit (B.sub.1) having an "A-directed" magnetization may be formed on the tracks. Data is recorded in accordance with the presence/absence of the bit (B.sub.1). A method of recording data in accordance with the presence/absence of the bit (B.sub.1) is called "bit position recording".
Note that a pit representing a minimum data unit area is also called a bit or mark.
In order to re-use a recorded medium, (1) the entire medium must be re-initialized at one time by an initializing device, (2) an erase head having the same arrangement as a recording head (but the erase head applies a magnetic field in a direction opposite to the recording head) must be added to a recording apparatus, or (3) as preliminary processing, a non-modulated laser beam must be radiated on the medium after a bias field is reversed by the recording apparatus.
Therefore, in the conventional magnetooptical recording method, it is impractical to perform an over-write operation, which can desirably form bits (B.sub.0) and (B.sub.1) according to new data regardless of the presence/absence of recorded data.
If the direction of a bias field Hb can be desirably modulated between the "A-direction" and "non-A direction" in units of bits (B.sub.0) and (B.sub.1), an over-write operation is possible. However, it is impossible to modulate the bias field Hb at high speed, and so, high-speed recording as a feature of the magnetooptical recording cannot be performed.
A magnetooptical recording method capable of performing an over-write operation by only modulating a beam intensity according to data without modulating the bias field Hb, i.e., without turning on/off the bias field or changing its intensity is disclosed in Japanese Laid-Open Patent Application No. 62-175948. The basic principle of an over-write method will be described below on the basis of this invention.
In the following description, an exchange coupling force .sigma..sub.w is referred to as an interface wall energy.
In this method, a laser beam is pulse-modulated according to data. In this case, a characteristic feature of this method is an intensity level of a laser beam. A bit (B.sub.1) having "A-directed" magnetization is formed at high level, and a bit (B.sub.0) having "non-A-directed" magnetization is formed at low level.